GEiyDLEY — GEOLOGY OF THE ISLE OF MAX, 
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or liglit-coloured limestone of Poolvash, and the lower or dark- 
coloured limestone of Derbybaven, etc. The list of the Manx Car- 
boniferous fossils upon which this subdivision of the Manx limestone 
into two groups is principally based, is of course very fiir from being 
complete; we have in our own collection several not included in it, 
and undoubtedly a more careful search would disclose many more ; 
while several designated in it as species have since been reduced to 
the condition of mere varieties. Yet, on the whole, it is a trust- 
worthy record, and the minute observations accompanying it are 
carefully made and generally accurate ; we feel, therefore, no diffi- 
culty in accepting this proposed division of the lower portion of the 
Manx Carboniferous limestone into at least two distinct groups. 
Subsequent to the deposition of the Poolvash beds a great change 
seems to have taken place in the physical condition of this part of the 
southern basin. The strata were violently disturbed along a line of 
fault parallel, or nearly so, with the present coast-line, and a consi- 
derable outburst of trap took place, which has greatly altered the 
underlying limestone, in some places converting it into pure dolomite. 
The extreme violence of this outburst, however, seems to have soon 
abated, after which the volcanic ash was poured out so quietly'- as 
not to interfere to any great extent with the ordinary operations of 
organic life — organic remains being found imbedded in it as regu- 
larly as in the limestone. At the same time that this submarine 
eruption was thus forming in one part of the basin a thick bed of 
volcanic ash, a deposit of black carbonaceous mud was also being 
formed in another part, and these two dissimilar sources by ming- 
ling have formed a very curious and interesting rock, one or other 
of the two ingredients predominating according to the varying cir- 
cumstances of the case. " At one period the carbonaceous deposit 
seems to have entirely prevailed : perhaps the volcanic action entirely 
ceased, gathering strength for a subsequent eruption. The bed then 
formed has its own lithological character and fossils ; it is the black 
Poolvash marble." After a time, however, this period of quiet de- 
posit was abruptly terminated, and the volcanic action renewed with 
increased violence ; the trappean and mixed beds already formed were 
violently broken up, reduced to a fragmentary condition, and mixed 
up with a fresh outpouring of volcanic matter ; and a sort of " trap 
breccia" was formed, in which the imbedded fragments of the older 
rocks are considerably altered by heat. 
Such is a brief and very imperfect account, agreeing mainly with 
Mr. Cummiug's, of a remarkably interesthig series of rocks over- 
lying the Manx Carboniferous limestone. Commencing near the 
Slack of Scarlet, a huge mass of columnar basalt, insulated at high 
water and forming the S.AY. horn of Castletown Bay, stretching 
along the south coast to Poolvash Bay, a distance of about two miles, 
they include, in addition to the imbedded limestone of pure or mixed 
origin, almost every variety of igneous trap, from the light porous 
ash to the hard columnar basalt, and present an appearance wild and 
rugged beyond description. 
YOL. Y. 2 a 
